Some previous studies have shown that the number of IgE receptors on human basophils correlates with circulating IgE level, and others suggest that atopy influences the binding of IgE to its receptor. This study proposes to characterize the IgE-IgE receptor binding on basophils of allergic and normal individuals by determining the number of receptors per cell and the in vivo association constant (Ko). These same binding characteristics will also be determined for other donors on basophils and mast cells from surgically removed lung samples in the same individual. In addition, in vitro shedding of IgE receptors will be studied on basophils and lung mast cells in cell culture. Finally, the possibility of in vivo induction of IgE receptors by circulating IgE will be investigated on rat mast cells.